Pulse width modulation (PWM) is a conventional technology used for controlling power converters to achieve output power, voltage, and current regulation. Conventional flyback power converters include a power stage for delivering electrical power from a power source to a load, a switch in the power stage that electrically couples or decouples the load to the power source, and a switch controller coupled to the switch for controlling the on-time and off time for the switch. The on-time and off-time for the switch can be modified by the controller based on a feedback signal representing output power, voltage, or current. The energy is stored in the transformer core gap when the switch is on, and is transferred to the load circuit when the switch is off. Regulation can be accomplished by, among other things, measuring the output power, voltage, or current, and feeding an indicating signal back to the primary side controller, which can modify the tON-time and tOFF-time of the switch accordingly to effectively regulate the output power, voltage, or current.
In power supply designs, it is necessary to regulate power, voltage, or current depending upon the application. One type of regulation corresponds to the regulation of current in a mode called “Constant Current”, or (CC) mode.
What is needed is a primary side sensing current control system and a method that (1) controls the current limit such that it is maintained within a small range of any acceptable input voltage, e.g., 90 to 264 Volts RMS, and (2) causes the output voltage of the PWM controller to drop as the output load increase when the full current is reached, (3) uses the instantaneous value of the VIN to influence a cycle by cycle calculation of the on-time and off-time, thus allowing removal of the line frequency components from the output by compensating for them on a cycle by cycle basis in the time calculations, and (4) attenuates the ripple performance of the power supply during operation in Constant Current (CC) mode using, for example, this previously mentioned calculation method for generating on-time (TON) during CC mode operation.